<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
   <script>
   var a=1;
   a=(++a,a++)
   console.log(a);//2
   a=(a++,++a);
   console.log(a);//4 
   //逗号运算符，它将先计算左边的参数，再计算右边的参数值。然后返回最右边参数的值。
   </script> 
</body>
</html>